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DES-7200 Configuration Guide Chapter 5 DHCP Relay Configuration
5-1
5 DHCP Relay
Configuration
5.1 Overview
5.1.1 Understanding
DHCP
The DHCP protocol is widely used to dynamically allocate the reusable network
resources, for example, IP address.
The DHCP Client sends the DHCP DISCOVER packet in broadcast form to the
DHCP Server. After the DHCP Server receives the DHCP DISCOVER packet, it
allocates resources such as IP address to the the DHCP Client according to the
appropriate policy, and sends the DHCP OFFER packet. After the DHCP Client
receives the DHCP OFFER packet, it checks if the resources are available. If
resources are available, it sends the DHCP REQUEST packet. If not, it sends
the DHCP DISCOVER packet. When the DHCP server receives the DHCP
REQUEST packet, it checks if the IP addresses (or other limited resources) can
be allocated. If yes, it sends the DHCP ACK packet. If not, it sends the DHCP
NAK packet. When the DHCP Client receives the DHCP ACK packet, it starts to
use the resources allocated by the DHCP server. If it receives the DHCP NAK
packet, it may re-send the DHCP DISCOVER packet to request another IP
address.
5.1.2 Understanding the
DHCP Relay Agent
The destination IP address of DHCP REQUEST packet is 255.255.255.255.
This type of packets is only forwarded inside the subnet and is not to be
forwarded by the devices. For dynamic IP address allocation across network
segments, the DHCP Relay Agent is created. It encapsulates the received
DHCP REQUEST packet into unicast IP packets and forwards it to the DHCP
server. At the same time, it forwards the received DHCP response pacekt to the
DHCP Client. In this way, the DHCP Relay Agent works as a transit station
responsible for communicating with the DHCP Clients and the DHCP Server on